a. Field of Invention
The invention relates to optical imaging and, more particularly to an afocal optical lens formed of transmissive optical material (glass, polymer, etc.) with confocal paraboloids on the front and back surfaces and used to create a compact, lightweight, permanently-aligned, aberration-free afocal telescope.
b. Background of the Invention
Interplanetary communications systems can benefit significantly from the introduction of free-space optical communications systems in which pulses of light are used to encode data for the long journey back to Earth. Such optical systems have demonstrated their potential to increase data transmission rates by orders of magnitude compared to more traditional RF systems of similar mass and power. However, to succeed at this, the optical communications systems require high-quality telescopes of about 20-50 cm diameter to expand their beams, generating more concentration of pulse energy (and hence more efficient collection of that energy) at the receiver on Earth. The extreme conditions of the space launch environment make the development and alignment of such a telescope difficult. This is due to thermal excursions as the telescope moves in and out of planetary shadows, and the violent shaking of the telescope during launch, both of which are serious impediments to maintaining alignment of the various optical surfaces in the telescope. The consequences of such a misalignment are that the signal can be mis-directed away from the Earth-bound receiver system, or aberrated enough to spread the light out at the receiver and prevent detection, tracking and decoding of the interplanetary signal. As a result, telescopes intended for launch to space require stiff, heavy structural members to maintain alignment, and may employ active metering and control systems to center and precisely position the optical elements for optimal performance. These structural elements add significant dead weight to the instrument, and consume precious resources during design and testing their effectiveness and maintaining the integrity and alignment of the optical system through the harsh thermal, decompression, vibrational and mechanical shock environments encountered in a space-qualified system.
Frequently, optical instruments employ afocal telescopes (in which the final output beam from a distant source is neither converging nor diverging) to expand or compress the sizes of incoming signals or emitted laser beams in order to deal with them more easily. Compressing a beam allows subsequent beam-shaping and directing optics to be smaller, generating a savings in system weight and consumed volume. Additionally, the optical beam compression is inversely proportional to the system's angular magnification, allowing a low-resolution beam steering element to be used for more precise centering of the image. Traditional afocal telescope systems frequently include a front-surface primary mirror with a concave surface curved to exact shape, and a separate convex front-surface secondary mirror, designed to collimate the light from a distant point source.
Monolithic telescopes have been built, in which curved surfaces are figured onto the front and back sides of glass cylinders. See, “Optical Design for Two Telescopes” by Arthur S. DeVany, Applied Optics, p. 201 (February 1963). However, this design used spherical surfaces which severely limits its useful focal ratio. Moreover, it was not well corrected for aberrations, and was far too massive to be practical as a rugged space-based instrument.
For expansion or concentration of laser beams or narrow field-of-view optical systems, systems of nested confocal paraboloids are well known to have excellent imaging characteristics. Two on-axis confocal paraboloidal mirrors (a large concave reflector paired with a smaller convex reflector) can deliver a collimated beam of reduced diameter onto a curved Petzval surface. Such a system furthermore corrects for all primary (3rd order Seidel and chromatic) aberrations. See, Modern Lens Design by Warren J. Smith, pp. 295-296. However, previous beam expander designs use at least two discrete elements to generate the compression or expansion, and are quite sensitive to misalignment of the lens components from thermal changes or mechanical mistreatment. Thus, there remains a need for an afocal optical communications telescope which is lightweight and rugged, operates across a wide wavelength range, provides excellent stigmatic imaging, and is insensitive to thermal changes.
Disclosed herein is an afocal monolithic optical element formed of transmissive optical material (glass, polymer, etc.) into which confocal reflecting paraboloids are figured on the front and back surfaces to create an aberration-free afocal telescope. The combination of two highly-configured confocal paraboloids on opposite sides of a single piece of glass allows for correction of all primary aberrations, and opens their aberration-free use to the full optical, UV and IR spectrum of light.